In present 35 mm commercial theater films, the film frames are spaced center-to-center by 4 perforations (0.748 inch) and the film frames are stepped at 24 frames per second past the film gate of a theater projector, resulting in an average film speed of substantially 90 feet per minute. The film moves at the same speed past the sound head, where a thin slit of light passes through the soundtrack and is detected by a photocell. The output of the photocell is amplified and equalized, and used to drive the loud speaker system of a theater. It is generally accepted in the industry, that the sound should be faithfully reproduced at a frequency of up to 12.5 kHz (or 13 kHz).
In a new film format, described in U.S. Pat. No. 5,534,954, the film frames are spaced at 2.5 perforations (0.468 inch), which results in saving 37.5% of the film stock previously used. It is estimated that this would save over $100 million dollars per year in film stock cost for the motion picture industry. However, the film moves only 5/8ths or 62.5% as fast, or at substantially 56 feet per minute. This new film format has been referred to as CDP for Compact Distribution Print.
There has been concern in the industry that a film speed less than the present speed of 90 fpm, would result in degradation of higher frequency sounds reproduced from the soundtrack.
The concern of the industry was stated in an article in the prestigious Journal of the SMPTE (Society of Motion Picture and Television Engineers) in its December, 1988 issue. That article described problems that would be encountered if the industry went from 4 perforations (90 fpm) to 3 perforations (68 fpm). The article stated that the frequency response would be reduced by 25%, and that with standard equalization, the bandwidth would be reduced from 13 kHz to 6 kHz, and with increased amplification at higher frequency resulting in a noise penalty. It was stated that this (68 fpm) would result in a significant reduction in audio quality. The use of an even lower speed of 56 fps for the CDP, would presumably result in a further reduction in audio quality, which would prevent the acceptance of such a low film speed in commercial motion picture theaters. A sound system which used the conventional variable width film soundtrack, so current theater projector sound systems could be used with minimum changes, which faithfully reproduced sound at up to 12.5 kHz at minimal additional cost, would enable the widespread use of such low speed and a consequent great saving in film stock cost.